feat: custom branch name templates (#571)

* Add branch-name-template config option

* Logging

* Use branch name template

* Add label to template variables

* Add description template variable

* More concise description for branch_name_template

* Remove more granular time template variables

* Only fetch first label

* Add check for empty template-generated name

* Clean up comments, docstrings

* Merge createBranchTemplateVariables into generateBranchName

* Still replace undefined values

* Fall back to default on duplicate branch

* Parameterize description wordcount

* Remove some over-explanatory comments

* NUM_DESCRIPTION_WORDS: 3 -> 5

src/github/context.ts

@@ -88,6 +88,7 @@ type BaseContext = {
     labelTrigger: string;
     baseBranch?: string;
     branchPrefix: string;
+    branchNameTemplate?: string;
     useStickyComment: boolean;
     useCommitSigning: boolean;
     sshSigningKey: string;
@@ -145,6 +146,7 @@ export function parseGitHubContext(): GitHubContext {
       labelTrigger: process.env.LABEL_TRIGGER ?? "",
       baseBranch: process.env.BASE_BRANCH,
       branchPrefix: process.env.BRANCH_PREFIX ?? "claude/",
+      branchNameTemplate: process.env.BRANCH_NAME_TEMPLATE,
       useStickyComment: process.env.USE_STICKY_COMMENT === "true",
       useCommitSigning: process.env.USE_COMMIT_SIGNING === "true",
       sshSigningKey: process.env.SSH_SIGNING_KEY || "",